Background and purpose: Diabetic cardiomyopathy is a complication of diabetes defined as cardiac dysfunction without the involvement of pericardial vessels, hypertension, or cardiac valve disorders. Ranolazine, an antianginal drug, acts through blocking of cardiac late sodium channels and/or inhibiting betaoxidation of fatty acids. With regard to its mechanism of action, the present work has been carried out to investigate the potential useful effects of ranolazine on the systolic and diastolic dysfunctions in an experimental RAT model of diabetic cardiomyopathy. Lidocaine, as a sodium channel blocker, was used to have a clearer image of the involved mechanisms. Experimental approach: Diabetes was induced by streptozocin. After 8 weeks, the effects of cumulative concentRATions of ranolazine and lidocaine were evaluated on diabetic and normal HEARTs by the Langendorff method. Finally, the HEARTs were ISOLATED from the Langendorff system and adenosine three phosphates (ATP) and adenosine diphosphate (ADP) concentRATions were measured to assay the metabolic effect of ranolazine. Findings/Results: Ranolazine significantly decreased the velocity of systolic contraction (+dP/dt) and the velocity of diastolic relaxation (-dP/dt) and developed pressure in normal and diabetic RAT HEARTs. However, this negative effect was greater in normal HEARTs compared to diabetics. Ranolazine (100 µ M) decreased the ATP level only in normal HEARTs and the ATP/ADP RATio decreased significantly (P < 0. 05) in both groups. This reduction was more prominent in normal HEARTs. Conclusion and implications: It is concluded that in the ISOLATED RAT HEART prepaRATion, ranolazine has no benefit on diabetic cardiomyopathy and may even worsen it. It seems that these effects are related to the metabolic effects of ranolazine.